There are many types of protection mechanisms that may be used in connection-oriented packet switched networks. Such protection mechanisms aim to provide protection to a network in the event of failures of paths, links or devices within the network so that disruptions to communications services are minimised. A further aim of such protection mechanisms is to avoid loss of traffic in the event of failures within the network.
It is known to provide protection mechanisms in a network operating a Hierarchical Virtual Private LAN Service (H-VPLS) under the standard IETF RFC 4762. H-VPLS enables Ethernet multipoint-to-multipoint Layer 2 Virtual Private Network (VPN) services. A feature of H-VPLS that facilitates the known protection mechanisms is the ability of network devices to automatically discover and signal to other network devices an association with a particular VPN so that the network devices query and learn VPN associations. Such automated behaviour is achieved by using messages which are part of the Operations, Administration and Maintenance (OAM) capabilities embedded within Ethernet as described in the standard ITU-T G.808.1.
A problem associated with the existing protection mechanisms operating under H-VPLS is that they are prone to configuration errors due to the large number of OAM messages required to keep track of VPN associations. Furthermore, centralised operating mechanisms require the network devices to interrogate the centralised servers to learn VPN associations. These centralised operating mechanisms add additional management functions to the overall network which may place a heavy burden in terms of network management. Network providers are hence required to ‘spend’ resources in their network in the form of such network management.
A further problem with the known protection mechanisms offered within H-VPLS is that they do not provide a sufficient level of reliability to provide a “carrier class” service. Such a carrier class service is required by Next Generation Networks (NGN) and is defined in a measurable way to determine whether a particular technology has the required quality of service. Typically there are five categories in which a particular technology must perform well to be termed a carrier class service. These five categories include service requirements, network requirements, security requirements, operational requirements, and network design requirements. For example, determining a service requirement may include using a mathematical formula to calculate a measure of voice quality as a so-called “R-value” according to ITU-T G.107. The way in which the other categories are determined are well known and will not be described further.
Currently available protection mechanisms may not attain the standard required to provide a carrier class service. Such a carrier class service must be able to provide protection in the case of a multiple failure within the network such as a failure of part of a worker path and a protection path at the same time. Such a double failure may be relatively common because the worker and protection paths may be physically located in the same duct in the ground which may be repeatedly dug up and recovered in a metropolitan area to perform maintenance on electricity, gas, water or other services. Conventional 1+1 protection schemes, where traffic in the network is actively transmitted on both a worker path and a protection path and the receiver is responsible for deciding which traffic flow to use, cannot be used to provide protection in the event of a double failure. Furthermore, conventional 1:1 protection schemes, where the traffic is only transmitted on the working path, but a protection path is allocated and used for sending traffic when a failure occurs, also cannot be used to provide protection in the event of a double failure. Recovery from a double failure cannot even be performed using known Sub Network Connection Protection (SNCP).
What is needed is an improved way of providing protection mechanisms to a communications network to provide a carrier class service and to reduce the above-mentioned problems.